This is a question I’ve been mulling over for awhile, and it’s been surprisingly hard to get a definite answer to, but I’m increasingly thinking the answer is “yes.” One apparently decisive piece of evidence is City of God, Book XII, Chapter 10, which is titled, “Of the Falseness of the History Which Allots Many Thousand Years to the World’s Past.”
There’s a wrinkle here in that I’ve heard it suggested that the chapter headings in City of God were added by a later scribe/editor. I don’t know if there’s anything to that suggestion, but until I can find something definitive on that, the chapter heading isn’t conclusive for what Augustine thought about the age of the earth. Now the text of that section says:
They are deceived, too, by those highly mendacious documents which profess to give the history of many thousand years, though, reckoning by the sacred writings, we find that not 6000 years have yet passed.
But passed since what? The very first sentence of the chapter says:
Let us, then, omit the conjectures of men who know not what they say, when they speak of thenature and origin of the human race.
So maybe Augustine was merely claiming that the human race is less than 6000 years old. That Augustine thought that, by the way, is clear from Book XV, where Augustine makes clear that he takes the genealogies of Genesis, including the ages of various Biblical figures, literally. But maybe Augustine was leaving open the possibility of a view where the earth is old, it’s just the human race that’s young.
Wait a minute, though. The sort of modern old-earth creationism that accepts an old earth but tries to take as much of Genesis literally as possible is driven mainly by geological findings from the last two centuries. The idea of a few thousand year-old human race combined with an earth that’s millions of years would have been a really weird one for Augustine to have in his historical context. So already City of God looks like pretty strong, if not quite conclusive, evidence that Augustine was a young-earth creationist.
The reality is that On the Literal Interpretation of Genesis makes clear that while Augustine thought the Bible was chock-full of figurative meanings, he also thought a lot of it was to be taken literally, and that includes the story of Adam and Eve–a story which is hard to reconcile with modern science.
The book’s implications for the age of the earth are a little less clear, but on close inspection they seem to support Augustine as a young-earther. You see, Augustine definitely took the six days of creation in Genesis 1 non-literally… but the specific way he took them non-literally was to say they describe simultaneous events. And since the six days of creation in Genesis 1 covers the creation of the heavens and the earth and all life on it, including humans, that suggests humans were created at the same time the earth as a whole was. Combine that with a literalistic reading of the rest of Genesis, and we get a young-earth view.
I’m not quite sure this is right, and I’m hoping some of you among my readers will be able to confirm or disconfirm this (my readers are awesome that way). In any case, the fact that Augustine clearly had a literalistic reading of Genesis 2 onwards–and the fact that this was important for his doctrine of original sin–should debunk the idea that Augustine shows how wonderfully science-friendly Christianity is capable of being.